Today, many different wireless systems exist, ranging from indoor wireless LANs (Local Area Networks) to outdoor cellular systems. Generally, the numerous wireless systems are not compatible with each other, making it difficult to roam from one system to another. Although there have been attempts to unify third-generation wireless systems, incompatible systems are expected to co-exist in the future. Furthermore, wireless LANs and cellular wireless systems are being developed independently, and such systems are also evolving independently. So far, no wireless technology has emerged as a common and long-term universal solution.
IP (Internet Protocol), which is already a universal network-layer protocol for packet networks, is rapidly becoming a promising universal network-layer protocol for wireless systems. An IP terminal, with multiple radio interfaces, can roam between different wireless systems if they all support IP as a common network layer. Unlike today's wireless systems in which Radio Access Networks (RANs) are mostly proprietary, IP provides an open interface and promotes an open market. IP will also enable widely adopted and rapidly growing IP-based applications to run over wireless networks. Moreover, distributed, autonomous IP-based wireless base stations have the potential of making the wireless systems more robust, scalable, and cost effective. There is, as a result, a current trend towards the design of Internet Protocol (IP) centric wireless networks comprising a large number of cells each covered by an autonomous IP router base station.
There are, however, many challenges to realizing distributed all-IP wireless networks. For the sake of specificity in discussing these challenges as well as pointing out problem areas, reference is made to FIG. 1. The depiction of network 100 in FIG. 1 illustrates an exemplary configuration of a network that uses IP-based wireless base stations (designated BSs). The coverage area of the wireless network is defined by a multiplicity of cells (e.g., cells 101, 102, 103). The geographical area covered by each wireless base station is referred to as a cell (e.g., BS 111 serves cell 101, and so forth). When mobile station 104 moves from one cell (e.g., cell 101 originally) into the overlapping regions (e.g., overlap of cells 101 and 102) of the coverage areas of multiple base stations, base station 111 may perform a “hand-off” of mobile station 104 to base station 112. Hand-off is a process whereby a mobile station communicating with one wireless base station is switched to another base station during a session. Overlap regions 117 and 118 are coverage areas where hand-off is effected. For example, as mobile station 104 moves into region 117 while roaming in cell 101, the radio signal strength from BS 2 (depicted by reference numeral 115) may be greater than the radio signal strength (114) from BS 1, so hand-off is warranted to maintain the quality of the established session.
Among the key challenges in a distributed all-IP wireless network is how to support “soft hand-off”. As suggested above, hand-off is the process that allows a mobile station's session-in-progress to continue without interruption when a mobile station (MS) moves from one wireless cell to another. Soft hand-off is a form of hand-off whereby a mobile station can start communication with the target base stations without interrupting the communication with the serving base station. Thus, soft hand-off allows a MS to communicate with multiple base stations (BSs) simultaneously. In particular, soft hand-off has been shown to be an effective way for increasing the capacity, reliability, and coverage range of CDMA-based wireless networks. Soft hand-off also provides more time for carrying out the hand-off procedure.
Soft hand-off in a CDMA-based wireless system is the focus of the subject matter of the present invention. In Code Division Multiple Access (CDMA) radio systems, a narrowband user message signal is multiplied by a very large bandwidth signal called the spreading signal. The spreading signal is a pseudo-noise code sequence that has a communication signal rate, which is an order of magnitude greater than the data rate of the user message signal. All users in a CDMA system may transmit simultaneously. Each user has its own pseudorandom code for coding its own message signal—each code is approximately orthogonal to all other codes. A receiver is assigned a code to detect a desired user message signal, and performs a time correlation operation to detect only the specific assigned code. All other codes appear as noise due to de-correlation. CDMA is effective in wireless systems because a receiver can be assigned a multiplicity of codes to detect message signals from a corresponding multiplicity of transmitters, thereby engendering the soft hand-off process.
An IP router is an IP network device that runs IP layer routing protocol (e.g., OSPF and BGP) and forwards IP packets. The running of a routing protocol decides the “routing policy”, and the forwarding of IP packets realizes the “routing mechanism”. IP packets arriving from the wireline IP network (121) at a given base station (e.g., BS 111 over wireline path 122 or BS 112 over path 123) can be routed by the routing mechanism of the base station to mobile station 104 (or other appropriate wireline devices that connect directly to the base station).
The fact that mobile stations (MSs) in different cells belong to different IP subnets suggests that a CDMA MS has multiple IP addresses as well as changes at least one of these IP address every time it moves into a new cell. Apart from additional delay due to getting and changing of IP addresses, as well as possible modification of the end-to-end session between an MS and its corresponding host, having multiple IP addresses per MS makes soft hand-off more difficult to implement because an MS receives multiple IP packet streams from different BSs with different traffic loads. The contents of these IP packet streams coming to the MS from different BSs will not be synchronized because copies of the same packet on each of these streams either experience different delays at different BSs, or may be lost altogether before reaching the MS. Consequently, copies of the same packets from different base stations may not be correctly combined by the MS's radio system. As a matter of fact, in this case, it is also possible that the MS's radio system combines copies of different packets received from different BSs.
From another viewpoint, in today's circuit-switched CDMA networks such as IS-95, a centralized Base Station Controller (BSC) is responsible for data distribution in the forward direction (from BS to MS). The BSC creates and distributes multiple streams of the same data over layer-2 circuits to multiple BSs that in turn relay the data to the MS. The MS's radio system (typically working below the IP layer) collaborates with the BSs to synchronize the radio channel frames and combine the radio signals received from different BSs to generate a single final copy of received data. The BSC helps ensure data content synchronization by ensuring that the matching layer-2 frames sent to different base stations contain copies of the same data. In the reverse direction (from MS to BS), the MS ensures that the matching layer-2 frames sent to different BSs contain copies of the same data. The BSC then selects one of the frames received from different base stations as the final copy of the data.
One problem already alluded to is loss of data content synchronization. With distributed BSs, centralized control entities, such as the BSC in circuit switched wireless networks, will no longer exist. Consequently, even though the CDMA radio system is capable of synchronizing the link and physical layer frames on the radio channel, it cannot, on its own, guarantee that the matching frames from different base stations will carry copies of the same data. For example, IP packets can be lost on their way to the MS, creating random gaps in the packet streams received by the MS from different BSs. Furthermore, copies of the same data may arrive at the MS at different times due to the random delays suffered by the packets. Random gaps and delays can lead to a loss of data content synchronization. Suppose that packet X is lost at BS 1 (due to, for example, buffer overflow) but is not lost at BS 2. Then, another totally unrelated packet Y from BS 1 and packet X from BS 2 may arrive at the MS at the same time and the MS's radio system will not be able to tell that they are not copies of the same data and will hence erroneously combine X with Y.
The art is devoid of a methodology and concomitant systems that effect soft hand-off in an all-IP wireless network that uses autonomous BSs in a configuration having the following characteristics that differentiate the configuration from existing wireless networks: (a) the BSs use IP protocols for both signaling and transport of user traffic. For example, they may route/forward IP packets based on information carried in the IP headers, perform IP-layer signaling, mobility management and Quality of Service (QoS) management functions; (b) the BSs function autonomously. There is no centralized signaling and control over the behaviors of the BSs; (c) the BSs are interconnected via an IP network, which could have arbitrary network topology such as bus, ring, star, tree, etc.; and (d) the cells (a cell is a geographical radio coverage area of a BS) can be arranged in any arbitrary configuration.
In order to support soft hand-off and satisfy its stringent synchronization requirements, current CDMA RANs usually take the following measures. A CDMA RAN provides means of stringent synchronization among its elements. The BSC manages the transmission power of MSs and BSs to ensure low error rate as well as minimize the power consumption. Moreover, as the MS moves, the BSC interacts with it to select and maintain an “optimum” set of BSs with which the MS remains in contact. On the forward link the BSC receives packets destined for the MS segments and assembles them into radio frames, and replicates the radio frames and transmits copies to BSs that are currently in contact with the MS. On the reverse link, the BSC collects copies of the radio frames received from BSs that are currently in contact with the MS, selects one of them, and synthesizes IP packets for forwarding to the wireline backbone.
Today's CDMA networks use BSCs as the focal anchoring points for frame distribution and selection as well as content synchronization during soft hand-off. However, IP-centric networks do not employ a BSC and as such there is no focal anchoring point for control of soft hand-offs. However, without a BSC process to control the transmissions from the Base Stations, the soft hand-off process in a IP-centric CDMA environment suffer from: (i) the lack of a means for efficient distribution of packet flows to multiple BSs, (ii) loss of content synchronization because the multiple packet flows destined for the MS experience different loss and delay across the network, (iii) inaccuracy of signal combination at the radio (i.e. physical) layer caused by the combination of radio signals at the physical layer of the MS resulting in erroneous synthesis of packets at higher layers because contents of their packet flows within received signals are not synchronized, and (iv) the need to have the MS select and maintain their active set of BSs.
Thus there is a need in the art for a soft hand-off process to be used in IP centric CDMA networks that overcome the limitations of the prior art as set forth above.